The present invention relates to a power line communication apparatus and a data relay method, in which an electronic device having a communication function can be connected to a transmission path using a power line as well as a communication interface such as an Ethernet (Registered Trademark), and data can be transmitted between the electronic device and the transmission path.
For example, a system capable of easily implementing communication operations in a house by communicatably connecting a plurality of devices including an information device such as a personal computer and a variety of electric devices such as a television set, a recorder device, a video reproducing device, and an Internet protocol (IP) telephone with one another on a predetermined communication network has been proposed. When a wired data communication is implemented in a house, typically, a cable used as a transmission path or a wire line including connectors should be provided in necessary areas. Therefore, various construction processes are necessary when a communication system is established.
In a house, a commercial electric power voltage, such as AC 120V (60 Hz) or 100V (50/60 Hz) is used in most of the cases. Therefore, a power line for supplying this commercial electric power is already provided in every region in a house. This power line can be used as a transmission path for data communication by connecting a communication device to an electric socket of a commercial electric power to obtain a transmission path without installing a separate wire line for data communication.
Such a power line communication (PLC) technology for using the power line in communication is disclosed in Japanese Patent Application Publication No. 2000-165304. In the present, some manufacturers are studying or developing this PLC technology for a predetermined frequency band (such as 1.7 MHz to 80 MHz in U.S. and 2 MHz to 30 MHz in Japan). Specifically, it has been conceived that multi-carrier signals are generated using a plurality of sub-carriers as in an orthogonal frequency division multiplexing (OFDM) method and transmitted through a power line.
In addition, the electric devices having a communication function based on an Internet protocol typically use an Ethernet (Registered Trademark) interface as a standard communication interface. Therefore, when a communication network is constructed by using a power line as a transmission path in a house, it is necessary to provide a bridge unit for relaying data transmission between a power line and an Ethernet (Registered Trademark) communication interface. In this case, in order for the bridge unit to perform communication through the power line, the bridge unit should have an internal modem unit (i.e., a PLC bridge) for power line communication. Otherwise, the bridge unit should have an external modem unit (i.e., a PLC modem unit) for power line communication.
However, a power line wiring in an indoor environment is abnormally complicated, and a wiring condition is significantly different from each building. Therefore, performance of the power line as a transmission path is significantly different in every place in the indoor environment. Furthermore, since types of electric devices connected to this power line are also different, various noises and variations in impedance may possibly occur. For this reason, when communication is performed via the power line, a desired communication rate may not be obtained, or a communication quality may be degraded due to the reduced signal-to-noise (S/N) ratio in comparison with a dedicated wired transmission path.
Accordingly, in the power line communication, a transmission path is predicted in a predetermined timing before or during the communication on a transmission path between a transmit terminal and a receive terminal, transmission path conditions (i.e., transmission path characteristics) such as the S/N ratio are measured in order to set transmission parameters such that a maximum transmission rate (i.e., a bit rate) can be obtained with an allowable range. In this case, as a transmission parameter, modulation factors (i.e., a data duplication rate) of each carrier in a multi-carrier signal are determined. When the condition of the transmission path is satisfactory, a data transmission amount within a unit time interval is increased (i.e., a bit rate is increased) by increasing the modulation factor. On the contrary, when the condition of the transmission path is not satisfactory, the data transmission amount within a unit time interval is reduced (i.e., the bit rate is reduced) by decreasing the modulation factor. As a result, it is possible to reduce an error rate during communication under a predetermined value.
Meanwhile, when various electric devices are connected with one another on a network in a house to transmit streaming data such as video or audio data, it is necessary to guarantee a quality of service (QoS) in order to prevent loss of data.
However, according to a conventional power line communication apparatus, there was no means for guaranteeing the QoS, and it was impossible to obtain a time slot by combining the power line communication with other communication interfaces such as Ethernet (Registered Trademark). Therefore, it was impossible to guarantee a satisfactory QoS when electric devices are connected to transmit the streaming data. As a result, errors such as loss of data may occur.
As described above, in a conventional power line communication apparatus, there was no means for guaranteeing the QoS, and it was impossible to obtain the time slot by combining the power line communication with other communication interfaces such as Ethernet (Registered Trademark). Therefore, it was impossible to guarantee a satisfactory QoS when the connected electric devices transmit streaming data. In addition, it was impossible to allocate the time slot to each communication line for connecting a predetermined electric device or control the priority.